U.S. Pat. No. 3,209,280 for Atomic Storage Cell For Atomic Hydrogen Maser by Robert Vessot and Norman Ramsey and U.S. Pat. No. 3,255,423 for Atomic Hydrogen Maser, both by Norman Ramsey and Daniel Kleppner and U.S. Pat. No. 3,388,342 for Atomic Hydrogen Maser With Enlarged Atom Storage Container by Norman Ramsey disclose a hydrogen gas maser atomic clock. Prior art hydrogen gas masers useful with atomic clocks operate on the hyperfine transition frequency f.sub.0 of atomic hydrogen, which is 1.42040575168 gigahertz. Such hydrogen gas masers provide extremely accurate clock standards because the hyperfine transition of ground state atomic hydrogen has a very narrow line width of the order of 1 hertz. If the hydrogen hyperfine transition is not externally stimulated, losses in the maser cavity must be sufficiently low such that the power radiated by the atoms would overcome cavity losses and oscillation can be sustained. Such low-loss narrow band width cavities have a relatively large size of the order of the wavelength of the radiation or 21 cm. The low loss cavity required to sustain oscillation is not necessary if the hydrogen is externally stimulated. In some prior art hydrogen gas maser clocks, the atoms are stimulated to radiate by continuous wave stimulation with a phase modulated resonant radiation synthesized from a voltage controlled crystal oscillator (VCXO), typically at 5 MHz. The radiation of the hydrogen gas in the maser is coherently detected and an error voltage proportional to the frequency difference between the atom transition frequency and the frequency of the stimulating signal is used to servo the VCXO. The VCXO output thus serves as a hydrogen atom resonance controlled frequency standard. One difficulty with such prior art devices is that the coherent emission from the hydrogen must be sensed in the presence of the stimulating signal. As a result, frequency pulling may occur whenever the amplitude of the stimulating signal drifts. Furthermore, phase drifts in the sensing electronics will introduce systematic errors. Significant frequency pulling is also produced when the cavity resonant frequency drifts out of tune.
Another disadvantage in such prior art devices is that the frequency synthesizer used to generate the continuous wave stimulating signal must have high resolution and stability in its output. Fluctuations both in frequency and amplitude in the synthesizer output will degrade the clock performance. Thus, it is apparent that the accuracy of such prior art atomic clocks place stringent stability requirements on the electronics.